FIG. 1 shows a prior art standard bipolar gain stage with poor power supply rejection. The prior art circuit of FIG. 1 includes transistors Q.sub.1 -Q.sub.3, negative rail V.sub.SS, positive rail V.sub.DD, input voltage V.sub.IN, output voltage V.sub.OUT, and bias voltage V.sub.B1. Some prior art techniques such as emitter degeneration and cascoding can be used to help the power supply rejection of the device of FIG. 1, but do not allow the gain stage to swing as closely to the power supply rail.
FIG. 2 shows how emitter degeneration can be used to increase the power supply rejection at the expense of headroom. The prior art circuit of FIG. 2 includes transistors Q.sub.1 -Q.sub.3, negative rail V.sub.SS, positive rail V.sub.DD, input voltage V.sub.IN, output voltage V.sub.OUT, bias voltage V.sub.B1, and resistors R.sub.0 and R.sub.1. FIG. 3 shows how cascoding can also be used to increase the power supply rejection, again with a reduction in headroom. The prior art circuit of FIG. 3 includes transistors Q.sub.1 -Q.sub.3, negative rail V.sub.SS, positive rail V.sub.DD, input voltage V.sub.IN, output voltage V.sub.OUT, bias voltage V.sub.B1, and transistors Q.sub.4 and Q.sub.5. Other types of current mirrors can be used in place of the current mirror in FIG. 3 to improve the power supply rejection, but each of them will remove at least an additional v.sub.be drop from the headroom over the standard gain stage shown in FIG. 1.